Flight report — April 18, 2026

UC Davis launches two high-altitude balloons on Picnic Day

A student-built tracker reached 13.9 km on the morning flight before drifting out of receiver range. The afternoon flight stalled at roughly 5 km. Both flights were tracked exclusively from a single ground station on the UC Davis campus.

Two balloons were launched from the UC Davis campus on Saturday, April 18, 2026, the day of the university’s annual Picnic Day public event. Both flights transmitted Horus Binary v3 telemetry under the payload callsigns UCDAV1 and UCDAV2. Telemetry was received and uploaded to the SondeHub Amateur tracker by a single station, callsign UCDAVIS, located at 38.5425, -121.753 — on or near the campus Hoagland Hall area. A third callsign, UCDAV3, appears in SondeHub records from the early morning hours before the launches and shows characteristics consistent with a stationary bench-test session rather than a flight.

The flights are presumed to be the work of the UC Davis Space and Satellite Systems (SSS) Club. The club’s own LinkedIn description states that in 2026 it began a “High-Altitude Balloon (HAB) educational program to introduce more UC Davis students to the skills and processes involved in engineering group work while also supporting the REALOP [CubeSat] mission via a simulated testing approach.” The callsign pattern and the on-campus ground station location are consistent with that program. The SSS Club has not separately confirmed these specific flights to this report.

The flights

Telemetry contents

The UCD packets use the Horus Binary v3 modulation but a custom payload schema rather than the standard 22-byte format. Two packet lengths were observed: a 48-byte short format (the majority of packets) and a 96-byte long format (transmitted occasionally). Decoded fields that appear consistently in the packets:

• Position — latitude, longitude, altitude, ground speed
• Frame counter
• Ascent rate
• GPS satellite count and fix-status flags
• Battery voltage
• External temperature, humidity, and pressure
• Internal temperatures (multiple, including a custom-named pair)

The longer-format packets carry additional fields with names suggesting internal housekeeping (stats_1_0/1/2, temps-heat_2_0/1/2/3) and a second set of temperature readings. The meaning of these additional fields is not documented in any public source available for this report. The packets contain 48 or 96 bytes of decoded payload, but the team’s full payload schema definition has not been published, so a portion of each packet’s bytes cannot be interpreted without reference to the team’s source code.

Receive coverage

Every packet uploaded to SondeHub for all three callsigns (UCDAV1, UCDAV2, UCDAV3) came from a single uploader: UCDAVIS, located at the Hoagland Hall area on the UC Davis campus. The receive station was running Horus-GUI version 0.6.1.1 with a Nooelec NESDR Smart RTL-SDR. No packets from any other receiver appear in the public SondeHub record for these payloads on April 18.

The receive frequency is not present in any uploaded packet. This is a known characteristic of the Horus-GUI software, which does not include the SDR’s tuned dial frequency in its uploads to SondeHub. The headless horusdemodlib tool, by contrast, does include this field. As a result, the SondeHub tracker page for these payloads shows “Receiver-Reported Payload Transmit Frequency: No data,” and other Horus Binary receivers in the region had no public information about which frequency to monitor.

What can be inferred from the data

The telemetry indicates UCDAV2’s payload was operating normally and climbing at a healthy rate when its signal was lost from the campus receiver. A balloon at 13.9 km drifting east at 60 km/h moves out of line-of-sight from a low-elevation ground station within tens of kilometers, especially as it heads toward terrain. Whether the balloon continued to a normal burst altitude after telemetry was lost is not known from the SondeHub record.

UCDAV1’s data tells a different story. Ascent rate decayed smoothly from healthy values to zero over many minutes — a pattern more consistent with a balloon-side issue (under-fill, slow leak, or partial envelope damage) than a sudden tracker failure. The GPS dropout in the final two packets is consistent with a payload that stopped ascending and may have begun swinging or descending under a degraded balloon. Whether the balloon was recovered, and at what location, is not known.

UCDAV3’s data fits the pattern of a final pre-launch verification of the telemetry chain, conducted in the early morning hours before the morning flight. The frame-counter resets, sensor manipulation, and stationary location all support that interpretation.

Background

Compiled from SondeHub Amateur telemetry packets for UCDAV1, UCDAV2, and UCDAV3, captured April 18, 2026. All flight outcomes and telemetry contents described above are direct observations of the uploaded packet data. Operator identity (UC Davis SSS Club) is supported by the club’s own public description of its 2026 HAB program and by the ground station location on the UC Davis campus; these specific flights have not been separately confirmed by the club.